In order to ensure confidentiality of or authentication of messages transferred between a pair of correspondents in a data communication system, it is usual to employ one or more cryptographic protocols to secure the message. Such protocols must be capable of withstanding a variety of attacks that may be made by an interloper in an attempt to break the protocol. One such attack is a replay attack. “Replay attacks” attempt to replicate an action performed by an earlier transmission to obtain information about the correspondents private information or keys used in the encryption by recording messages and reproducing the protocol at a later date.